This invention relates to an improved apparatus for use with a tubing string used in conducting drill stem tests of oil and gas wells. More particularly, the apparatus relates to a check valve apparatus allowing trapped fluid to flow from the interior of the test string to the well annulus when the test string is being lowered in a well bore into sealing engagement with a wireline set production packer.
During the drilling of oil and gas wells, various types of drilling fluids known as muds are used to maintain formation fluids in intersected formations by virtue of their hydrostatic pressure. In order to allow the formation fluids to flow to the surface for analysis, it is necessary to isolate the formation to be tested from the hydrostatic pressure of the drilling fluid in the well bore. This is done by lowering a tubular pipe string comprising testing tools and piping or testing string, as it is commonly known, to the formation to be tested, then sealing the well annulus between the testing string and above the formation with a packer.
Typically, a tester valve is included at the lower end of the testing string and is lowered in the closed condition such that a lower pressure exists in the bore of the testing string. After the formation is isolated from the well annulus, the tester valve is opened to lower the pressure in the well bore adjacent the formation to be tested such that formation fluids may flow from the formation into the lower end of the tubular string and from there to the surface.
Pressure sensors are typically included in the testing string such that the tester valve may be opened and closed and pressure recordings made to evaluate the production potential of the formation being tested.
Two types packers may be used. The first type is a packer which may be incorporated in a testing string and expanded by manipulation of the testing string to effect a seal between the walls of the well bore and the tubular pipe testing string. A second type is a wireline set production packer which is lowered and attached to the walls of the well bore at the desired location. The testing string having a seal assembly at its lower end is subsequently lowered into the well bore until the seal assembly is seated in the production type packer to effect the seal necessary to isolate the formation.
It will be understood that if a production type packer is used, fluid trapped in the well bore below the production packer will be compressed as the testing string is further lowered into place after the seal assembly has effected its seal in the production packer. This fluid trapped in the well bore below the packer must be displaced back into the formation as the seal assembly is further lowered into the packer. The displacement of drilling fluid into the formation is undesirable in that it may seal or otherwise damage the pore spaces in the formation through which oil and gas must be produced. Also, if an annulus pressure operated well tester valve having a pressure operated isolation valve such as that disclosed in U.S. Pat. No. 3,964,544 or U.S. Pat. No. 3,976,136 is used, the compression of fluid in the central bore of the testing string below the tester valve will increase the operating pressure of the tester valve to an undesirably high level.
The use of the present invention prevents excessive pressure from the trapped fluid from developing which might otherwise damage the packer, the pressure recorder, the tester valve, or other tools in the testing string. Also, this trapped fluid might support the testing string thereby preventing its downward movement to completely seat in a tubing hanger. When a tester valve in the testing string is subsequently opened, the trapped fluid will be released allowing the testing string to fall which may, in turn, damage the tubing of the string or the hanger.